Retrospectives:

1) The original site and book are online in full at http://cluetrain.com and http://cluetrain.com/book

2) The 10th anniversary edition has new chapters by the four original authors, plus additional ones by JP Rangaswami, Dan Gillmor and Jake McKee.

3) David Weinberger and I posted an addendum to Cluetrain in 2015 called New Clues: http://cluetrain.com/newclues

4) The word "cluetrain" is more or less constantly mentioned on Twitter: https://twitter.com/search?q=cluetrain

5) A search in Google books https://www.google.com/search?tbm=bks&q=cluetrain brings up more than 13,000 results, almost nineteen years after the original was published.

6) A search in Google Scholar https://scholar.google.com/scholar?en&q=cluetrain brings up more than 4,000 results.

7) A dig through old emails just turned up the earliest evidence  (at least to me) of Cluetrain's inception: a draft of a joint JOHO (David Weinberger's email list) and EGR (Chris Locke's list) posting, vetted for input by yours truly. This was when the three of us were first sharing the co-thinkings that became Cluetrain in early 1999. That email is dated 30 October 1998, meaning that more than two decades have passed since this thing started.